1. Field of the Invention
The present invention relates to a flow rate sensor for outputting a signal in response to a flow rate of a fluid being measured, and relates to a flow rate sensor suitable for measuring an intake air flow rate of an internal combustion engine in an automobile, for example.
2. Description of the Related Art
Generally, in an automotive engine, etc., an air-fuel mixture including fuel and intake air is burnt in a combustion chamber in the engine body, and rotational output from the engine is extracted from the resulting combustion pressure, requiring that the intake air flow rate be detected in order to calculate the injection rate, etc., of the fuel with high precision.
Thus, the flow rate sensor shown in Japanese Patent Non-Examined Laid-Open No. 2000-2572, for example, is known as a conventional technique of this kind.
FIG. 6 is a longitudinal section showing a conventional flow rate sensor such as described in Japanese Patent Non-Examined Laid-Open No. 2000-2572, for example, mounted to a main passage, FIG. 7 is a partial perspective showing an assembly process for the conventional flow rate sensor, FIG. 8 is an enlarged partial longitudinal section of the conventional flow rate sensor in FIG. 6, and FIG. 9 is a cross section taken along line IXxe2x80x94IX in FIG. 8 viewed from the direction of the arrows.
In the figures, a main passage 1 is formed into a cylindrical shape from, for example, a resin material, a metal material, etc., a small-diameter cylindrical mounting aperture 2 being disposed so as to project radially outward, and a passage forming body 3 having a vertically-aligned rectangular body being disposed so as to project radially inward from an inner wall surface of the main passage 1. A bypass passage 4 is formed into a general U shape inside this passage forming body 3, an inflow aperture 5 of the bypass passage 4 opening onto the vicinity of the axial center of the main passage 1 on a front surface of the passage forming body 3, and an outflow aperture 6 of the bypass passage 4 opening onto the main passage 1 on a lower surface of the passage forming body 3. In addition, an element insertion aperture 7 is formed in the passage forming body 3 at a position facing the mounting aperture 2.
A flow rate sensor 10 is constituted by a casing 11, a mount plate 18, a circuit board 21, a flow rate detecting element 23, etc.
The casing 11 is formed into a stepped cylindrical shape from a resin material, for example, and is constituted by: a collar-shaped mount portion 12 formed on a base end portion of the casing; a circuit accommodating portion 13 formed into a generally rectangular overall box shape and is disposed so as to extend to a first side of the mount portion 12; and a connector portion 14 formed on a second side of the mount portion 12, the connector portion sending and receiving signals to and from an external portion. A circuit board mount recess portion 15 surrounded by a peripheral wall 15a forming a rectangular shape, a mount plate interfitting groove 16 formed by cutting away a portion of the peripheral wall 15a at an extremity of the casing 11, and interfitting apertures 17 formed so as to be positioned on first and second sides of the mount plate interfitting groove 16 are disposed in the circuit accommodating portion 13.
The mount plate 18 is formed into a plate-shaped body from a metal material, for example, being composed of: a circuit board mount portion 19 formed by bending edge portions of the mount plate 18 on the left and right in FIG. 6; and an element mount portion 20 formed integrally at an extremity of the circuit board mount portion 19. A rectangular element accommodating recess portion 20a for accommodating the flow rate detecting element 23 is formed in this element mount portion 20. This mount plate 18 is mounted to the casing 11 by housing the circuit board mount portion 19 inside the circuit board mount recess portion 15 such that the element mount portion 20 fits into the mount plate interfitting groove 16. Here, an extremity of the element mount portion 20 projects from the casing 11.
The circuit board 21 is disposed on the circuit board mount portion 19, electronic components for sending and receiving electric signals to and from the flow rate detecting element 23 being mounted to the circuit board 21. First circuit board terminals 21a of the circuit board 21 and connector terminals 14a of the connector portion 14 are each electrically connected by first bonding wires 22a. 
The flow rate detecting element 23, as shown in FIG. 7, is provided with: a rectangular silicon substrate 24; a heater resistor 25 formed on a surface of the silicon substrate 24; a pair of temperature-detecting resistors 26 formed on the surface of the silicon substrate 24 so as to be positioned to the left and right of the heater resistor 25; and a temperature-compensating resistor 27 formed on the surface of the silicon substrate 24, the flow rate detecting element 23 being disposed inside the element accommodating recess portion 20a. Second circuit board terminals 21b of the circuit board 21 and element terminals 23a of the flow rate detecting element 23 are each electrically connected by second bonding wires 22b. 
Moreover, the heater resistor 25, the temperature-detecting resistors 26, and the temperature-compensating resistor 27 are electrically connected to each of the element terminals 23a by a wiring pattern (not shown) formed on the surface of the silicon substrate 24. Furthermore, the electronic components mounted to the circuit board 21 constitute a heater control circuit for controlling the heater resistor 25 of the flow rate detecting element 23, an amplifying circuit for amplifying detection signals from each of the temperature-detecting resistors 26, a reverse-current sensing circuit, etc.
A stopper member 28 is constituted by a stopper main body 29 and an elastic protrusion 30. The stopper main body 29, as shown in FIG. 7, is formed by: an elongated plate portion 29a extending flatly so as to lie across the mount plate interfitting groove 16; interfitting protrusions 29b positioned on left and right sides of the elongated plate portion 29a so as to project toward the interfitting apertures 17 of the circuit accommodating portion 13 and fit into the interfitting apertures 17; a central protrusion 29c positioned between the interfitting protrusions 29b so as to fit into the mount plate interfitting groove 16 and, as shown in FIG. 8, extend to a position in proximity to the second bonding wires 22b; and a stopper recess portion 29d formed between the elongated plate portion 29a and the central protrusion 29c. The elastic protrusion 30 is composed of a flexible elastic material such as silicone rubber, for example, and is fixed to a leading edge portion of the central protrusion 29c. The stopper member 28 is mounted to the casing 11 such that the interfitting protrusions 29b fit into the interfitting apertures 17. Here, the elastic protrusion 30, as shown in FIG. 8, is placed in contact with a surface of the flow rate detecting element 21 in an elastically-deformed state.
A sealant 31 is formed from a silicone gel, for example, and is injected inside of circuit board mount recess portion 15, as shown in FIGS. 6 and 8, so as to cover the surface of the circuit board 21, the bonding wires 22a and 22b, and the connector and element terminals 14a and 23a. Hence, short-circuiting of the bonding wires 22a and 22b is prevented and the electronic components mounted to the circuit board 21 are protected.
A cover body 32 is mounted to the casing 11 such that a peripheral portion thereof is fixed by adhesive to the peripheral wall 15a of the circuit board mount recess portion 15 and the stopper main body 29. Hence, the circuit board mount recess portion 15 is sealed over, and the stopper member 28 is held with the elastic protrusion 30 placed in contact with the surface of the flow rate detecting element 21 in an elastically-deformed state.
The flow rate sensor 10 constructed in this manner is mounted so as to project inside the main passage 1 from the mounting aperture 2. At this time, the element mount portion 20 of the flow rate sensor 10 is inserted inside the element insertion aperture 7, and the flow rate detecting element 23 is disposed inside the bypass passage 4.
This main passage 1 is connected partway along an air intake pipe of the engine, an air cleaner (not shown) being connected to a first end thereof, and an air intake manifold communicating with the inside of cylinders of the engine (not shown) being connected by means of a throttle valve, etc., (not shown) to a second end. Air cleaned by the air cleaner flows through the inside of the main passage 1 from right to left in FIG. 6, is directed inside the bypass passage 3 through the inflow aperture 5, flows over the surface of the flow rate detecting element 23 (the silicon substrate 24), then flows out into the main passage 1 through the outflow aperture 6.
A heating current which flows through the heater resistor 25 is controlled by a circuit constructed on the circuit board 21 such that the average temperature of the heater resistor 25 is higher than the temperature of air detected by the temperature-compensating resistor 27 by a predetermined amount. Hence, the flow rate of the air is detected by making use of the cooling effect the flow of air exerts on the heater resistor 25 and changes in the resistance values of each of the temperature-detecting resistors 26.
Because the conventional flow rate sensor 10 is constructed in the above manner, the stopper member 28 has a complex three-dimensional shape and the portion of the stopper member 28 placed in contact with the flow rate detecting element 23 is minute. Thus, one problem has been that forming the elastic protrusion 30 composed of an elastic material such as a silicone rubber, etc., only on the portion of the stopper member 28 to be placed in contact with the flow rate detecting element 23 requires that the dimensions of the elastic protrusion 30 be controlled with high precision and that the stopper main body 29 and the elastic member 30 be aligned with high precision, causing mass production to deteriorate, thereby increasing production costs.
The central protrusion 29c and the recess portion 29d which are formed into the stopper main body 29 have the effect of preventing pressure resulting when the sealant 31 is injected inside the circuit board mount recess portion 15 from acting directly on the elastic protrusion 30. However, air accumulates inside the recess portion 29d easily when the sealant 31 is injected. When air pockets form in the recess portion 29d, the second bonding wires 22b are partially exposed, and so another problem has been that short-circuiting occurs between the second bonding wires 22b. 
Thus, to eliminate these air pockets, a process has been required for removing the air from the air pockets by evacuating the ambient atmosphere from the casing 11 during the injection of the sealant 31 or before heat curing of the sealant 31, and so another problem has been that the number of work processes is increased, raising production costs.
As shown in FIG. 9, slight gaps 33 arise between the elongated plate portion 29a of the stopper main body 29 and the mount plate interfitting groove 16. A silicone gel is generally used for the sealant 31. This silicone gel is initially a liquid and becomes a gel on heat curing. Consequently, even if the gaps 33 are ideally reduced by controlling the dimensions of the elongated plate portion 29a and the mount plate interfitting groove 16 with high precision, the liquid silicone gel applied to cover the circuit board 21, etc., leaks out easily through the gaps 33 during heat curing. In addition, when a large amount of air flows through the main passage 1, pressure in the main passage 1 drops, giving rise to a pressure difference between the inside of the circuit board mount recess portion 15 sealed over by the cover body 32 and the inside of the main passage 1, and the silicone gel in cured gel form is sucked out through the gaps 33 due to this pressure difference and leaks out.
The flow rate detecting element 23 is accommodated inside the element accommodating recess portion 20a and secured by an adhesive to the element mount portion 20, but an epoxy adhesive is generally used for the adhesive bonding the flow rate detecting element 23 to the element mount portion 20. Many epoxy adhesives of this kind contain amine substances as curing agents or catalysts. On the other hand, xe2x80x9caddition-reactionxe2x80x9d silicone gels which cure by an addition reaction between vinyl groups and silane (SiH) groups using a platinum catalyst are used for the sealant 31. In that case, since the amine substances contained in the epoxy adhesive coordinate more strongly with the platinum catalyst than the vinyl groups during the heat curing of the silicone gel (the sealant 31), the amine substances act to inhibit curing of the silicone gel. As a result, the silicone gel in the region coming into contact with the epoxy adhesive is not cured, but instead remains in an oily state. Thus, silicone gel in oil form leaks out from the gaps 33 during and after curing of the silicone gel.
xe2x80x9cCondensation-reactionxe2x80x9d silicone rubbers which generally cure by a condensation reaction with moisture contained in the air are used for the elastic member 30. These condensation-reaction silicone rubbers contain organometallic salts and organic peroxides. When the addition-reaction silicone gel used as the sealant 31 is heat cured, the organometallic salts and the organic peroxides contained in the silicone rubber act to inhibit curing of the silicone gel (the sealant 31) since the organometallic salts and the organic peroxides coordinate more strongly with the platinum catalyst than the vinyl groups. As a result, the silicone gel in the region coming into contact with the silicone rubber is not cured, but instead remains in an oily state. Thus, silicone gel in oil form leaks out from the gaps 33 during and after curing of the silicone gel.
In order to ensure gel properties, the silicone gel initially contains a comparatively large amount of oil components not contributed to curing. As a result, the oil components leak out from the gaps 33 during and after curing of the silicone gel.
Hence, in the conventional flow rate sensor 10, the silicone gel and the oil components contained in the silicone gel leak out through the gaps 33 and adhere to the flow rate detecting element 23.
Because this kind of flow rate sensor is a thermosensitive flow rate sensor making use of heat transfer characteristics by which heat generated in the heater resistor 25 is lost to the fluid being measured (here, the air) from the surface of the flow rate detecting element 23, another problem has been that the heat transfer characteristics are changed significantly by the silicone gel and the oil components contained in the silicone gel adhering to the flow rate detecting element 23, making accurate flow rate detection impossible.
Silicone gel which has adhered once is heat cured by the heat from the heater resistor 25, adhering firmly to the flow rate detecting element 23. Thus, another problem has been that the flow rate detection characteristics of the flow rate sensor are changed with the passage of time by the silicone gel which adheres firmly to the flow rate detecting element 23, making accurate flow rate detection impossible, and tracking by the detection signal of changes in the flow rate of the fluid being measured significantly deteriorates, making responsiveness as a flow rate sensor poor.
The present invention aims to solve the above problems and an object of the present invention is to provide a flow rate sensor enabling accurate flow rate detection and also enabling deterioration in responsiveness to be suppressed by preventing a sealant sealing an electrical connection portion from leaking out and adhering to a flow rate detecting element.
With the above in view, a flow rate sensor of the present invention includes a holder at a first end of which a flat detector auxiliary portion is formed integrally, a flat flow rate detecting element for detecting a flow rate of a fluid being measured and a circuit board to which a control circuit for controlling an electric current flowing to the flow rate detecting element is mounted. An element accommodating recess portion is formed in a major surface of the detector auxiliary portion. The flow rate detecting element is mounted to the holder so as to be housed inside the element accommodating recess portion such that a major surface of the flow rate detecting element is positioned in a common plane with a major surface of the detector auxiliary portion. A terminal is built into the holder such that a second end of the terminal is electrically connected to the circuit board. A first end of the terminal is positioned in a common plane with the major surface of the detector auxiliary portion and extends onto the detector auxiliary portion. A frame-shaped peripheral wall member is mounted to the holder so as to surround an electrical connection portion formed by electrically connecting an electrode portion formed at a first end of the terminal of the flow rate detecting element and the first end of the terminal. A heat-curing addition-reaction sealant is injected inside the peripheral wall member so as to embed the electrical connection portion. A bottom surface of the peripheral wall member is secured by bonding to the detector auxiliary portion and the flow rate detecting element by an elastic adhesive.
Therefore, because the chemical resistance and environmental tolerance of the sealant are superior and leakage of the sealant resulting from aging is prevented, a flow rate sensor enabling accurate flow rate detection and also enabling deterioration in responsiveness to be suppressed is achieved.
With the above in view, a flow rate sensor of the present invention includes a holder at a first end of which a flat detector auxiliary portion is formed integrally, a flat flow rate detecting element for detecting a flow rate of a fluid being measured and a circuit board to which a control circuit for controlling an electric current flowing to the flow rate detecting element is mounted. An element accommodating recess portion is formed in a major surface of the detector auxiliary portion. The flow rate detecting element is mounted to the holder so as to be housed inside the element accommodating recess portion such that a major surface of the flow rate detecting element is positioned in a common plane with a major surface of the detector auxiliary portion. A terminal is built into the holder such that a second end of the terminal is electrically connected to the circuit board. A first end of the terminal is positioned in a common plane with the major surface of the detector auxiliary portion and extends onto the detector auxiliary portion. A frame-shaped peripheral wall member is mounted to the holder so as to surround an electrical connection portion formed by electrically connecting an electrode portion formed at a first end of the terminal of the flow rate detecting element and the first end of the terminal. A sealant is injected inside the peripheral wall member so as to embed the electrical connection portion. The sealant is constituted by a heat-curing addition-reaction gel or rubber containing a fluorine resin as a major constituent.
Therefore, because the chemical resistance and environmental tolerance of the sealant are superior and leakage of the sealant resulting from aging is prevented, a flow rate sensor enabling accurate flow rate detection and also enabling deterioration in responsiveness to be suppressed is achieved.